Manhole covers for permanent manhole installations are in widespread use. Typically, manhole covers are constructed in the form of a very heavy circular iron or steel plate, which closely fits into a recess around the lip of the manhole. Such plates require special tools or relatively heavy machinery in order to remove them.
A locking manhole insert cover for suspending equipment beneath the cover is disclosed in the U.S. patent to Wisniewski U.S. Pat. No. 5,328,291. This patent discloses a safety cover for manholes which is to be placed beneath a conventional manhole cover. The safety cover disclosed in the Wisniewski patent includes a plurality of arms which are extended outwardly by rotating a central cam. These arms extend under a lip or flange of the main manhole cover to hold the safety cover in place. A provision is made for locking the arms in place with a padlock; so that the cover cannot be removed.
The U.S. patent to Kagstrom U.S. Pat. No. 4,101,154 discloses a cover for well holes or the like. This cover is equipped with a self-locking mechanism in it. The locking mechanism comprises a pair of diametrically opposed arms, which are pivotally attached to a central shaft extending downwardly from the center of the well cover. The arms have a length chosen to cause them to slidably engage the walls of the hole when the cover is placed over the hole. If an attempt is made to remove the cover, the arms tend to pivot outwardly to more tightly engage the sides of the hole, preventing removal of the cover. When removal is desired, a rod or key is inserted through an aperture in a central tube attached to the cover to engage the lower ends of the pivoting arms to move them out of engagement with the sides of the hole. When the rod is in place, the cover readily can be removed. The arms do not extend into the sides of the well hole, but act as a wedge if an attempt is made to lift the cover from the well opening. Gravity holds the arms in place in the wedging or holding position.
The U.S. patent to Frishauf U.S. Pat. No. 5,052,851 discloses an emergency maintenance hole cover in the form of an inflatable balloon. The configuration of the balloon is in the general shape of a tire or hollow disk connected by a central web. The external edge of the balloon is inserted into the upper edge of the maintenance hole with which it is to be used. The balloon then is inflated; and the resilient material out of which it is made engages the edges of the maintenance hole, preventing water contaminants, animals and the like from passing into the hole. Clearly, this is a temporary installation, which is not intended to remain in place for extended periods of time.
Another patent which is directed to a locking device for locking a cover in place over a repair hole is disclosed in the U.S. patent to Brewer U.S. Pat. No. 4,053,116. The cover of this patent is placed over a vertical exposed tubular pipe in the area of the excavation. A relatively large plate covers the excavation; and a downwardly extending leg on the underside of the cover extends into the exposed pipe. The leg includes an anchoring device which engages the wall of the pipe to hold the assembly in place. The plate, which is the primary cover over the opening, is relatively heavy, requiring construction equipment to lift it into place and to remove it when it no longer is needed.
The U.S. patent to Clarke U.S. Pat. No. 3,426,659 is directed to a road excavation cover which is anchored in place by means of a horizontally arranged jack attached to the underside of the cover. The jack is operated to wedge the ends of the jack into engagement with the sides of the excavation to hold the cover in place; so that it cannot readily be moved or dislodged. This is not a cover for a circular vertical hole.
It is desirable to provide a manhole cover which may be made of lighter weight material than conventional covers, and which readily may be locked in place to secure the manhole against unauthorized exposure while permitting positive and easy release when desired.